Glucose isomerization enzymes catalyze the conversion of glucose (dextrose) to fructose in the manufacture of High Fructose Syrup (HFS), i.e. a syrup containing a mixture of glucose and fructose. All described glucose isomerases fundamentally are xylose isomerases. Thus, for the sake of clarity, the enzyme of this invention ordinarily will be identified as xylose isomerase, although the most important application thereof is for the isomerization of glucose to fructose.
Due to the high sweetening power of fructose and the low tendency to crystallize HFS is widely used in industry, where it can replace liquid sucrose and invert syrup in many food and beverage products such as soft-drinks, baked goods, canned fruit, ice cream, confectionery, jams and jellies.
A large number of patents are directed to different microbial sources of xylose isomerase. Many bacteria, e.g. species belonging to the genera Streptomyces; Actinoplanes: Bacillus: and Flavobacterium, and fungi, e.g. species belonging to the class Bacidiomycetes, have been identified in the patent literature as xylose isomerase producing microorganisms, vide e.g. U.S. Pat. No. 4,687,742 and EP patent publication No. 352,474.
For many years the wish has existed to perform the isomerization process at elevated temperatures, vide e.g. U.S. Pat. Nos. 4,410,627; 4,411,996; and 4,567,142. The conversion of glucose to fructose is a temperature dependent equilibrium reaction: The higher the temperature, the higher the yield of fructose at equilibrium. However, the isomerization temperature strongly affects the enzyme activity and stability. Higher temperatures will result in higher enzyme activity (increased glucose conversion rate), but also increased thermal denaturation of the enzyme. Therefore, the stability, and hence the productivity, decreases with higher temperatures. On the other hand, lower isomerization temperatures will lead to increased risk of microbial infection. The infection risk is minimized when operating at temperatures of from approximately 60.degree. C.
Of outstanding commercial interest are syrups containing approximately 55% (w/w dry substance) fructose. At this fructose level the syrup attains equal sweetness with sucrose on a weight to weight dry basis, and is used interchangeably as a total or partial replacement for sucrose in many food products, and especially in carbonated soft drinks.
Regards to thermal stability and enzyme consumption greatly influence the isomerization conditions. Thus, in practise, due to the life time of the enzyme and the productivity, syrups containing more than approximately 42% fructose are obtained by non-enzymatic treatment of the enzymatically manufactured HFS. The non-enzymatical treatment comprises expensive chromatographical separation to obtain a syrup with the desired fructose concentration. Therefore, an extensive search for xylose isomerases that allow the isomerization process to be carried out at higher temperatures in favour of a higher fructose yield has taken place.
It is an object of the invention to provide a xylose isomerase that in respect to thermal stability is superior over any hitherto known xylose isomerase. Moreover, it is an object of the invention to provide a isomerization process for high yield glucose conversion.